This year's midterm elections officially kick off Tuesday with the Texas primaries, where voters will sift through jam-packed fields of Democratic candidates vying to take on vulnerable Republican members of Congress.

Here's what you need to know:

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Polls open at 7 a.m. CST and close at 7 p.m. CST.Democrats are running in all of Texas' 36 congressional districts for the first time in 25 years.Democrats have an edge in early voting, a notable, though limited, gauge of voter enthusiasm in what's been a reliably red state.Candidates must top 50% in their primary, or the top two finishers will head to a runoff election in six weeks.Incumbent Republican Sen. Ted Cruz and Democratic Rep. Beto O'Rourke are likely to win their respective primaries. CNN recently moved the Texas Senate race from Solid Republican to Likely Republican.

Here's a closer look at the six races to watch tonight:

Texas land commissioner: Incumbent George P. Bush -- the son of Jeb Bush and the last Bush in political office -- faces a primary challenge from former land commissioner Jerry Patterson. This is a race over the Alamo: Patterson has campaigned against what he calls Bush's failed stewardship of the site of the historic battle.

7th District House race: It was already a bare-knuckled brawl among the seven Democrats vying to take on Republican Rep. John Culberson for his Houston-area seat. Then the Democratic Congressional Campaign Committee got involved, dropping a bomb on one of the candidates, Laura Moser, and turning the race into something of a proxy for the larger war within the party. Emily's List-backed lawyer Lizzie Pannill Fletcher and attorney Alex Triantaphyllis are other candidates to watch in a race that could be headed for a runoff.

23rd District House race: Party figures have split over this contest to take on GOP Rep. Will Hurd in this border district between Iraq War veteran Gina Ortiz Jones, former federal prosecutor Jay Hulings and former Agriculture Department official Judy Canales.

32nd District House race: The third competitive Republican-held seat in Texas is represented by Republican Pete Sessions. The Democrats vying to replace him are former NFL player and attorney Colin Allred, former Agriculture Department official Lillian Salerno and former State Department official Ed Meier.

Texas Democratic governor's primary: Senate candidate Beto O'Rourke, running to oppose Republican Sen. Ted Cruz, is the statewide headliner for Democrats this cycle. But Republican Gov. Greg Abbott is up for re-election as well, and former Dallas County Sheriff Lupe Valdez and businessman Andrew White are vying to take him on.

29th District Democratic race: The campaign to replace retiring Rep. Gene Green in this reliably blue district has seen an interesting split between Senate Minority Leader Chuck Schumer, who is backing Tahir Javed, and Emily's List and the other New York senator, Kirsten Gillibrand, who are backing state Sen. Sylvia Green.